For Roger Federer, on the first day of the French Open, there was a horrible ring to the belief that top seeds are never more vulnerable at a Grand Slam than in the first round.

He went out here and at Wimbledon at this stage last year and Monday he was the victim of another major opening-round upset when he was beaten 7-6 (8-6), 6-2, 7-6 (7-3) by Luis Horna, the Peruvian ranked 88th in the world whose performances had hardly caused a ripple since he was in the junior final here six years ago.

The way Federer returned a backhand tamely into the net on match point reflected his lack of initiative and ambition in a match that Horna won, at least in part, because he kept the points alive until the 21-year-old fifth seed added to a growing total of unforced errors.

It has been said of Federer that he has too many gilded shots in his repertoire for his own good. He arrived in Paris having won more matches (38) than any other player on the men’s tour and three titles — in Marseille, Dubai and Munich — on three different surfaces.

His success in the Hamburg Masters last year and in reaching the final in Rome earlier this month had demonstrated his ability to play at the highest level on clay, even though he had implied — possibly too often for his own good — that this was the surface on which he was most likely to struggle.

As 1998 Roland Garros champion Carlos Moya observed after he had recovered his confidence in time to wear down the Italian lucky loser, Filippo Volandri, 7-6 (9-7), 4-6, 6-2, 6-3, “he [Federer] has a game to play well anywhere, against anyone.”

Now that Federer has lost in the first round here three times in five visits and three times out of four at Wimbledon, where he believes he has his best chance of winning a Grand Slam, one is bound to wonder if the problem is temperament rather than talent.

He was ready when asked if he was beginning to worry about under-performing in the really big tournaments. “As long as you guys are not giving me a hard time, it’s okay,'' he said, unconvincingly.

While Horna, father of a month-old daughter, was celebrating what he called “the most exciting moments in my life and my career,” Federer was left to reflect on what might have been.

Tactically he had taken his cue from watching on television as Felix Mantilla tied up Federer in a web of astute but ultimately tedious groundstrokes while beating him in the Italian Open final.

Federer led 3-0 in the first set Monday and served for it at 5-4.

However, the stress was increasingly evident in his face and his gait from the moment he wasted a forehand on set point at 5-4 in the tiebreak, before compounding the error by missing what should have been an easy smash at 6-7.

“It wasn’t just the smash that left me disappointed, it was the way I had lost the set before that,” he said. “The overhead just added to the whole thing.”

Federer had almost feared trouble. When asked over the weekend to name his first seeded opponent in this tournament, he had replied: “You guys have to relax a bit, because I’m not playing a bad guy in the first round.”

“Now I’m sitting here trying to explain,” he added Monday.

The only disappointment for an admiring crowd when Andre Agassi, the 1999 champion, outclassed Karol Beck, of Slovakia, 6-2, 6-3, 6-3 on Centre Court was that he double-faulted on his first two match points before completing victory on his fourth. However, the errors in no way diminished the ovation he commanded at the end.

One in the wings to keep an eye on is 29-year-old former champion Yevgeny Kafelnikov, whose career has been extended by varicose vein surgery and who trounced Frenchman Julian Boutter 6-1, 6-2, 6-4.